I  The  Promontorio  Silver-Mine,  Durango, 
Mexico. 


KRANCIS  CHURCH  L.INCOLN, 

NEW  YORK,  N.  Y. 


A  Paper  Read  before  the  American  Institute  op  Mining 
Engineers,  at  the  Toronto  Meeting, 
July,  1907. 


AUTHOR'S  EDITION. 
19  0  8. 


subjeW  to  eevision. 


[TRANSACTIONS  OF  THE  AMERICAN  INSTITUTE  OF  MINING  ENGINEERS.] 

< 

The  Promontorio  Silver-Mine,  Durango,  Mexico. 

BY  FRANCIS  CHURCH  LINCOLN,*  NEW  YORK,  N.  Y. 
(Toronto  Meeting,  July,  1907.) 

I.  Situation  and  Surroundings. 

The  Promontorio  mine  is  situated  at  the  northern  end  of 
the  Sierra  San  Francisco  de  Coneto,  in  the  town  of  Promon- 
torio, Partido  of  El  Oro,  State  of  Durango,  Mexico.  As  shown 
in  the  sketch-map,  Fig.  1,  the  nearest  railroad  station  is  Chin- 
acates  on  the  Mexican  International  Railroad,  82  miles  north 
of  the  city  of  Durango.  The  mine  is  16  miles  north  of  the  sta- 
tion by  air-line.  It  is  reached  by  means  of  a  good  wagon-road 
which  first  crosses  the  Guatimape  Plain,  passing  the  ranches 
of  San  Antonio  and  San  Julian,  to  Estacion — a  distance  of 
about  15.5  miles — and  then  entering  the  mountains  crosses  the 
Sierra  to  Promontorio — a  further  distance  of  11.5  miles- 
making  in  all  a  distance  of  27  miles  by  road  from  railroad  to 
mine. 

Promontorio  is  just  beyond  the  summit  of  the  range,  at  an 
elevation  of  about  8,000  ft.  above  sea-level,  or  1,350  ft.  above 
the  Chinacates  station.  The  Castillo  de  San  Francisco,  the 
highest  peak  in  the  Sierra,  has  an  altitude  of  10,000  ft.,  and 
the  Promontorio  road  crosses  the  summit  of  the  ridge  at  an  ele- 
vation of  9,000  ft.  by  a  pass  just  below  this  peak  known  as  the 
Puerto  del  Almagre.  The  Promontorio  mill  is  situated  at  Santa 
Ines,  2  miles  by  tram  or  3  miles  by  wagon-road  to  the  north  of 
Promontorio  and  about  600  ft.  lower. 

The  Sierra  San  Francisco  de  Coneto  decreases  in  height 
towards  the  north  and  west  till  it  comes  to  an  end  at  the  Mel- 
chor  Arroyo  on  the  ranches  of  Melchor  and  Pamos.  Beyond 
this  arroyo  another  range,  the  Sierra  de  la  Candela,  begins. 
In  the  vicinity  of  the  Promontorio  mine  the  hills  are  very 
rugged  and  the  surface  is  furrowed  by  ravines  which  contain 
running  streams  during  the  rainy  season — July,  August  and 

*  Fellow  in  Geology,  Columbia  University,  New  York,  N.  Y. 

[1] 


84         THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO. 


September — but  are  dry  throughout  the  rest  of  the  year.  Fig. 
2  is  a  view  of  the  Promontorio  mine,  looking  east,  showing  the 


4%      Longitude     West  105 

%\       (              ^  ^ 
\c    (  Estacion 
\  \  San  Julian 

Santiago  \^^^  Antonio^ 
Papasquiaro%,  >.-(v"^ 

so'     from  XGreenwicli 

Ramos/  v^o-^ 
( 

'te'  if  (/santa  Ines 

PROMONTORIO 

\   Pi^^ '  m  ®  Potrillos 

Chinacat 
N  \ 

1  \ 

s 

Scale  1  =1,200,000 

Canatlan  ^ 

>t 

50l 

Ot 
>I 

o| 

DURANGO  ®'^24°M. 

Fig.  1.— Map  of  Part  or  Burango,  Mexico. 

Eefugio  shaft-house  in  the  ravine,  the  San  Joaquin  dump  on 
the  hill,  the  administration  buildings  on  the  left,  and  the  power- 

[2] 


THE   PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO.  85 


house  on  the  right.  Fig.  3  is  another  view,  looking  northward. 
The  nearest  permanent  flowing  water  is  the  Melchor  Arroyo,  a 


good-sized  stream  which  might  be  utilized  for  power.  A  con- 
siderable number  of  large  pine-trees  are  still  standing  on  the 
slopes  of  the  hills  immediately  adjacent  to  the  mine,  while  a 
short  distance  away,  oaks  and  other  small  trees  are  fairly 

[3] 


86  THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO. 


abundant  as  well,  promising  a  readily  available  supply  of  mitie- 
timbers  and  tire-wood  for  a  long  time  to  come. 


II.  The  Country-Eock. 

The  country-rock  of  Promontorio  is  a  rhyolite-porphyry. 
The  ground-mass  is  glassy,  showing  flow-lines,  and  the  pheno- 
crysts   are    large    quartzes  and   orthoclases,  together  with 

[4] 


THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO.  87 

smaller  and  somewhat  altered  hornblendes.  Small  grains  of 
magnetite  are  rather  plentiful,  and  a  much  lesser  number  of 
little  crystals  of  pyrite  are  to  be  seen.  The  rock  is,  therefore, 
a  typical  rhyolite-porphyry,  and  undoubtedly  belongs  to  the 
Tertiary  rhyolites  so  common  throughout  the  Sierra  Madre  of 
Mexico.  ; 

Of  special  interest  are  the  inclusions.  The  principal  ones 
are  small,  dark,  angular  fragments,  less  than  an  inch  in  diameter, 
which  are  distributed  rather  plentifully  in  some  parts  of  the 
porphyry.  Microscopic  examination  shows  these  inclusions  to 
be  fragments  of  andesite  and  dacite,  which  were  evidently 
broken  from  older  rocks  and  brought  up  from  a  depth  by  the 
rhyolite-porphyry.  Occasional  rounded  pieces  of  binary  granite, 
from  several  inches  to  a  foot  in  diameter,  are  also  to  be  ob- 
served in  the  porphyry.  These  may  have  been  broken  off 
from  underlying-rock  like  the  andesite  and  dacite ;  or  they 
may  be  simply  segregations,  since  they  are  composed  of  quartz 
and  orthoclase,  the  commonest  phenocrysts  of  the  porphyry. 
According  to  Ordonez,^  the  succession  of  Tertiary  eruptives  in 
Mexico  is  similar  in  all  districts,  and  may  be  summarized  as 
follows  : 

Group  No.  6,  Basalts — Andesitic  basalts.  ; 

Group  No.  5,  Dacites — Andesites. 

Group  No.  4,  Ehyolites. 

Group  No.  3,  Andesites — Dacites. 

Group  No.  2,  Diorites— Diabases. 

Group  No.  1,  Granites — Granulites. 

If  this  order  is  correct,  the  Promontorio  rhyolite-porphyry  be- 
longs to  group  ^o.  4.  The  dark  inclusions  in  this  porphyry 
show  that  group  l^o.  3  is  represented  in  depth  by  both  andesite 
and  dacite ;  the  binary  granite,  if  not  a  segregation,  is  repre- 
sentative of  group  1^0.  1 ;  but  inclusions  indicating  the  ex- 
istence of  group  ITo.  2  have  not  been  discovered. 

The  Promontorio  country-rock  has  a  well-developed  joint 
structure.  It  is  split  into  sheets  which  range  from  less  than 
an  inch  to  more  than  a  foot  in  thickness  and  are  of  con- 
siderable length  and  breadth.    The  strike  of  these  sheets  ik 


1  Las  Rhyolitas  de  Mexico,  Boletin  del  Instituto  Geologico  de  Mexico,  No.  14,  p. 
66  (1900). 

[5] 


88  THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO. 


about  ^N".  20°  W.  (magnetic),  and  their  dip  nearly  vertical  but 
inclining  slightly  towards  the  northeast. 

The  only  other  rock  in  the  vicinity  of  Promontorio  is  a  rhyo- 
lite  which  caps  the  range.  At  Coneto,  28  miles  SE.,  there  is 
an  outcrop  of  andesite  of  similar  character  to  that  of  the  in- 
clusions in  the  porphyr3\ 

III.  The  Vein. 

The  Promontorio  vein  strikes  through  the  rhyolite-porphyry 
N.  55°  W.  (magnetic).  Its  dip  is  vertical  at  the  surface,  inchn- 
ing  towards  the  SW.  in  depth.  Thus  it  cuts  the  joint-planes  of 
the  porphyry  at  an  acute  angle  in  both  strike  and  dip.  The 
Promontorio  vein  proper  has  been  followed  beneath  ground 
for  a  horizontal  distance  of  2,660  ft.,  but  it  cannot  be  traced 
so  far  upon  the  surface  because  of  the  covering  of  soil.  Below 
ground  the  vein  still  continues  towards  the  SE.,  while  to  the 
NW.  it  forks,  and  the  West  vein,  proceeding  from  the  hang- 
ing-wall of  the  Promontorio  vein  proper,  has  been  followed  an 
additional  distance  of  968  ft.  The  "West  vein  can  be  traced  on 
the  surface  much  farther,  and  is  probably  identical  with  the 
LaLuz  vein  tunneled  in  a  prospect  far  to  the  'NW.  of  Pro- 
montorio. To  the  SE.  no  vein  has  been  discovered  which  can 
be  identified  with  the  Promontorio  vein.  Either  the  vein  has 
pinched  out  rapidly  in  that  direction,  or,  as  seems  much  more 
likely,  the  vein  is  older  than  the  rhyolite  which  makes  its  ap- 
pearance to  the  SE.  and  has  been  covered  by  it. 

There  are  no  parallel  veins  near  the  Promontorio,  but  there 
are  numerous  cross-courses,  all  of  which  fault  the  E"W.  part  of 
the  Promontorio  vein  toward  the  SW.,  the  horizontal  displace- 
ments varying  from  1  to  13  ft.  These  cross-courses  are  in 
some  instances  simple  faults,  while  in  others  they  have  become 
mineralized  and  constitute  veins.  The  most  noteworthy  ex- 
ample is  the  Yeta  Dolores,  which  strikes  K  42°  E.  (magnetic) 
and  can  be  traced  as  a  well-defined  quartz-vein  all  the  way 
from  Santa  Ines  to  its  junction  with  the  Promontorio  vein — 
2  miles — and  for  some  distance  beyond.  Another  important 
cross-course  is  known  as  the  Yeta  Atravesada.  It  is  much  less 
marked  than  the  Yeta  Dolores,  strikes  K  28°  E.  (magnetic) 
and  is  possibly  identical  with  the  Los  ^aufragos  vein  which 
has  been  explored  in  a  prospect  to  the  SW.  of  Promontorio. 

[6] 


THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO.  89 


All  the  cross-courses  dip  steeply  to  the  SE.,  which  makes  their 
junctions  with  the  Promontorio  vein  pitch  steeply  in  this  direc- 
tion also. 

IV.  The  Ore. 

The  Promontorio  vein  is  frequently  divisible  into  three  dis- 
tinct parts:  1,  a  hanging-wall  portion  of  vein-matter;  2,  an 
intermediate  portion  of  more  or  less  altered  country-rock,  and 
3,  a  foot-wall  portion  of  vein-matter.  The  principal  value  of 
the  ore  is  in  silver,  and  sometimes  one,  sometimes  another,  por- 
tion of  the  vein  is  richest.  In  general,  however,  either  the 
foot-  or  the  hanging-wall  portion  contains  the  most  silver,  and 
the  intermediate  portion  is  most  likely  to  he  ore  when  both 
foot-  and  hanging-wall  portions  are  rich. 

Thus  the  Promontorio  ore  consists  of  vein-matter  and  min- 
eralized country-rock.  The  characteristics  of  the  fresh  coun- 
try-rock have  already  been  described.  It  has  been  mineralized 
in  two  ways:  1,  by  silicification  and  impregnation  with  small 
scattered  grains  of  the  same  sulphides  as  are  found  in  the 
vein-matter ;  and  2,  by  the  precipitation  of  secondary  minerals 
in  joint-cracks  and  decomposed  spots. 

The  common  primary  vein-minerals  are  quartz,  galena,  and 
sphalerite,  less  pyrite,  a  very  little  chalcopyrite,  and  minute 
quantities  of  bornite,  chalcocite,  and  covellite.  The  rare  pri- 
mary vein-minerals  are  tetrahedrite,  chalcocite,  argentite,  and 
native  gold.  The  oxidized  vein-filling  consists  of  quartz,  kao- 
lin, hematite,  wad,  and  limonite,  with  occasional  films  of  mala- 
chite and  linarite  and  remains  of  the  sulphides.  The  min- 
erals which  have  contributed  to  the  secondary  enrichment 
are  native  silver,  chalcocite,  and  a  little  chalcopyrite.  The 
native  silver  does  not  contain  even  a  trace  of  gold.  Secon- 
dary enrichments  occur  both  in  oxidized  portions  of  the  vein 
and  in  the  country-rock  of  the  walls  and  horses. 

Considering  the  frequency  with  which  free  silver  is  still  en- 
countered in  the  Promontorio  mine,  the  rather  even  grade  of 
the  ore  is  somewhat  surprising.  In  a  systematic  sampling  of 
the  mine,  the  highest  result  obtained  was  263.6  oz.  of  silver 
per  ton,  and  only  12  out  of  1,059  assays  exceeded  150  oz.  per 
ton.  The  ratio  by  weight  of  gold  to  silver  in  the  shipping-ore 
varies  from  2  : 1,000  to  3  :  1,000  parts.    The  presence  of  small 

[7] 


90  THE  PROMONTORIO  SILYER-MINE,  DURANGO,  MEXICO. 


amounts  of  copper-minerals  always  indicates  a  higli  silver-con- 
tent, but  in  all  cases  where  neither  copper-minerals  nor  native 
silver  can  be  identified  in  the  vein-filling,  assays  are  necessary 
to  distinguish  between  ore  and  waste. 

When  the  assays  are  plotted  upon  the  mine-map,  it  is  seen 
that  the  ore  is  arranged  in  shoots  which,  like  the  junctions  be- 
tween the  Promontorio  vein  and  cross-courses,  all  pitch  steeply 
toward  the  SE.  These  shoots  sometimes  parallel  the  faults, 
while  in  other  cases  they  are  cut  by  the  faults  or  occur  in 
unfaulted  parts  of  the  vein.  They  are  usually  long  and  nar- 
row, extending  from  15  to  100  ft.  along  a  level  and  cutting 
many  levels  on  their  pitch.  After  continuing  downwards  for 
a  number  of  levels,  shoots  sometimes  pinch  out.  'New  shoots 
may  come  in  along  the  line  of  the  old  ones,  or  make  their  ap- 
pearance in  intermediate  positions. 

The  shoots  are  either  primary  or  secondary,  both  pitching  in 
the  same  general  direction.  The  primary  shoots  are  distinguish- 
able by  their  comparatively  high  content  of  sulphides,  by  their 
lack  of  secondary  minerals,  and  by  their  habit  of  being  cut  off 
by  faults  unless  occurring  in  unfaulted  parts  of  the  vein. 
On  the  other  hand,  the  secondary  shoots  are  recognizable 
b}^  their  low  content  of  sulphides,  by  the  presence  in  their 
richer  portions  of  the  secondary  minerals,  native  silver,  chalco- 
cite,  and  chalcopyrite,  and  by  their  tendency  to  follow  closely 
well-defined  faults.  The  primary  ore-shoots  are  dominant  in 
the  lower  levels,  the  secondary  in  the  upper.  The  secondary 
ore-shoots  reach  their  maximum  development  in  the  neigh- 
borhood of  the  fourth  level,  where  the  Yeta  Dolores  shoot  ex- 
tends horizontally  for  460  ft.  and  the  Yeta  Atravesada  shoot 
for  456  ft. 

i^one  of  the  cross-veins  have  developed  ore  at  a  distance  from 
the  Promontorio  vein,  although  small  amounts  of  ore  have  been 
taken  from  some  of  them  near  their  junctions  with  the  Promon- 
torio vein.  Considerable  prospecting  has  been  carried  on,  but 
no  other  mine  has  been  discovered  in  the  district. 


[8] 


THE  PROMOXTORIO  SILYER-MIXE,  DURAXGO,  MEXICO.  91 


Y.  The  Eelatioxs  or  the  Primary  Veix-Mixerals. 
Polished  surfaces  of  the  sulphide  ore  were  prepared  aud  ex- 
amined microscopically  under  the  direction  and  after  the 
methods-  of  Dr.  William  Campbell,  for  whose  assistance  I  de- 
sire to  express  my  thanks.  The  specimens  consisted,  for  the 
most  part,  of  three  minerals,  galena,  sphalerite,  and  quartz, 


Fig.  4. — Low-Geade  Syxphide  Op.e.    Olagnified  80  times; .    Eelatioxs  of 
Sphaeeeite,  Qeaetz,  axd  Gaee^'a. 

whose  relations  are  illustrated  in  Fig.  4.  This  shows  that  the 
sphalerite  was  formed  first.  It  was  fissured,  and  subsequently 
quartz  was  deposited  in  the  fissures,  and  also  in  crystals  on 
the  exterior.  Finally  came  galena,  filling  the  vugs  in  the 
quartz-vein  and  molding  itself  about  the  previously  deposited 
sphalerite  and  quartz. 

-  Economic  Geology,  toI.  i.,  pp.  751  to  "56  (1906). 
[9] 

UNIVERSITY-  OF 
ILUNOIS  LIBRARY 
AT  URBANA  CHAMPAIGN 


92 


THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO. 


Pyrite  is  an  important  component  of  the  sulphide  ore,  though 
by  no  means  so  plentiful  as  sphalerite,  galena,  or  quartz.  The 
relation  of  pyrite  to  these  minerals  is  clearly  indicated  in  Fig.  5. 
Pyrite  is  there  shown  to  be  incrusted  with  sphalerite,  which 
is,  in  turn,  incrusted  with  quartz-crystals,  while  galena  occupies 
all  the  remaining  space,  just  as  it  does  in  Fig.  4.  Pyrite,  there- 
fore, clearly  antedates  sphalerite,  and  was  the  first  vein-mineral 
to  be  formed. 


Fig.  5. — Low-Ghade  Sulphide  Ore.    (Magnified  80  times).    Eelations  of 
Pyrite,  Sphalerite,  Quartz,  and  Galena. 

Chalcopyrite  is  a  much  rarer  constituent  of  the  primary  ore. 
Its  usual  mode  of  occurrence,  illustrated  in  Fig.  6,  is  in  grains 
attached  to  the  sphalerite  and  surrounded  by  galena.  In  this 
illustration  one  grain  is  shown  surrounded  by  bornite  and  an- 
other is  intimately  associated  with  covellite.  In  other  speci- 
mens chalcocite  has  been  found  in  similar  relations  with  chal- 
copyrite. These  associations  indicate  that  the  minute  amounts 
of  bornite,  chalcocite,  and  covellite  found  in  the  ore  are  pro- 
bably secondary  and  derived  from  chalcopyrite.  Chalcopyrite 
has  not  been  found  contiguous  to  quartz,  but  since  it  is  entirely 

[10] 


THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO.  93 


absent  from  quartz-veins  in  sphalerite  and  present  in  galena- 
veins  in  this  same  mineral,  it  is  reasonable  to  infer  that  it  is 


Fig.  6.— Medium-Grade  Sulphide  Ore.   (Magnified  80  times).  Relations 
OF  Sphalerite,  Chalcopyrite,  Covellite,  Bornite,  and  Galena. 


Fig.  7.— Medium-Grade  Sulphide  Ore.    (Magnified  80  times).  Relations 
OF  Pyrite,  Quartz,  Bornite,  Chalcocite,  and  Galena. 


later  than  the  quartz.    The  secondary  copper-minerals  asso- 
ciated with  grains  of  pyrite  are  illustrated  in  Fig.  7,  which 
[11] 


94  THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO. 


shows  that  the  bornite  is  younger  than  the  quartz'  and  older 
than  the  chalcocite. 

The  order  of  succession  of  the  minerals  in  the  primary  ore 
of  Promontorio  is  therefore:  1,  pyrite ;  2,  sphalerite;  (period 
of  crushing) ;  3,  quartz ;  4,  chalcopyrite ;  5,  galena. 

Rich  primary  minerals  are  exceedingly  rare  at  the  Promon- 
torio mine.  They  play  no  recognized  part  in  the  mine's  pro- 
duction, although  they  are  probably  present  in  minute  amounts 
in  the  ordinary  primary  ore. 


Fig.  8.— High-Grade  Sulphide  Ore.    f  Magnified  200  times).    Keeations  of 
Quartz  Gold,  Tetrahedrite,  Chalcocite,  and  Argentite. 


A  specimen  of  rich  ore  obtained  from  the  fourth  level  is  ap- 
parently of  primary  origin.  It  consists  of  numerous  metallic 
gray  stringers  and  veinlets  in  milky  white  quartz.  Polished 
surfaces,  examined  under  the  microscope,  show  the  cavity- 
fillings  to  consist  of  tetrahedrite,  chalcocite,  argentite  and 
native  gold,  arranged  as  illustrated  by  Figs.  8  and  9.  (These 
minerals  were  identified  by  methods  which  I  developed  and 
shall  soon  publish.) 

Quartz  was  deposited,  then  shattered,  and  native  gold  de- 
posited upon  it  in  small  isolated  crystals.    Tetrahedrite  came 

[12] 


THE  PEOMONTORIO  SILVER-MINE,  DURANGO,  MEXICO.  95 

next,  forming  detached  ci^stals  also,  which  occasionally  sur- 
rounded gold.  A  crust  of  chalcocite  then  formed  over  all, 
and  what  gold  had  not  already  been  involved  in  tetrahedrite 
became  surrounded  by  chalcocite.  This  is  indicated  by  Fig.  9. 
The  section  of  veinlet  there  represented  was  completely  filled 
with  chalcocite,  but  in  most  instances  the  central  parts  of  the 
crevices  were  left  open  and  were  filled  later  with  argentite,  as 
shown  in  Fig.  8. 


Fig.  9. — High-Grade  Sulphide  Oe,e.    (Magnified  200  times).  Eelations 
OF  Quartz,  Gold,  and  Chalcocite. 


The  order  of  succession  in  this  rich  piece  of  ore  was  there- 
fore: 1,  quartz;  (period  of  crushing);  2,  native  gold;  3,  tetra- 
hedrite ;  4,  chalcocite ;  5,  argentite. 

It  will  be  noticed  that  the  period  of  crushing  came  just  after 
the  deposition  of  quartz  instead  of  just  before,  as  in  the  ordi- 
nary primary  ore.  The  difierence  in  conditions  thus  clearly, 
indicated  probably  caused  the  deposition  of  particularly  rich 
ore  at  the  point  whence  this  specimen  was  taken. 


[13] 


96         THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO. 

YI.  Geological  History  of  the  Promontorio  District. 

The  Promontorio  rhjolite-porphyry  was  extruded  during 
Tertiary  time  through  underlying  andesite,  dacite  and,  perhaps, 
binary  granite.  Pressure  during  cooling  developed  sheet- 
jointing. 

After  the  consolidation  of  the  porphyry  a  disturbance  took 
place  which  resulted  in  the  formation  of  an  extensive  N  W.-SE. 
fault,  through  which  heated  ore-bearing  solutions  rose.  This 
disturbance  was  probably  caused  by  an  intrusion  of  molten 
igneous  rock  that  faulted  the  overlying  layers  and  gave  forth 
plutonic  emanations,  which,  ascending  through  the  fault,  de- 
posited the  primary  ore  of  the  Promontorio  vein. 

The  conditions  were  such  that  first  pyrite,  then  sphalerite 
was  deposited.  A  slight  rearrangement  along  the  fault  led  to 
the  shattering  of  some  of  the  pyrite  and  sphalerite,  and  at  the 
same  time  brought  about  a  change  of  conditions,  so  that  quartz 
and,  soon  after,  chalcopyrite  were  formed.  Finally,  galena  was 
deposited  in  all  remaining  cavities,  and  the  deposition  of  pri- 
mary minerals  came  to  an  end. 

After  the  mineralization  of  the  Promontorio  fault  another 
upheaval  took  place,  which  was  probably  contemporaneous  with 
the  extrusion  of  the  rhyolite  that  caps  the  Sierra  San  Francisco. 
This  caused  the  formation  of  a  series  of  NE.-S  W.  faults,  sev- 
eral of  which  intersected  the  Promontorio  vein.  These  faults 
were  slightly  mineralized,  perhaps  by  lateral  secretion  from  the 
porphyry,  but  certainly  in  a  different  manner  from  the  Promon- 
torio fault. 

The  district  was  covered  by  rhyolite  for  a  while,  but,  when 
this  was  eroded,  the  only  partly  tilled  cross-faults  formed  con- 
venient channels  by  which  surface-waters  were  tapped  oft*  and, 
entering  the  Promontorio  vein  in  the  neigborhood  of  these 
cross-faults,  rearranged  its  contents.  Rich  deposits  of  second- 
ary minerals  were  thus  formed  in  the  vicinity  of  the  cross- 
courses. 

VII.  Mining. 

The  Promontorio  mine  was  discovered  by  Joaquin  Contreras 
in  1880,  and  purchased  by  its  present  owner,  the  i^Tegociacion 
Minera  de  Promontorio,  S.  A.,  in  1887.  It  is  at  present  the 
only  real  mine  in  the  whole  Sierra  San  Francisco  de  Coneto 
district.    It  should  be  noted,  however,  that  the  famous  Po- 

[14] 


THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO.  97 

trillos  tin-deposits  are  situated  in  these  mountains,  and  that  a 
system  of  silver-gold  veins  at  Coneto  has  been  worked  inter- 
mittently for  a  long  period  of  time. 

When  mining  began  at  Promontorio,  it  was  necessary  to 
freight  the  ore  260  miles  to  the  railroad  at  Fresnillo.  At  that 
time,  shipping-ore  had  to  contain  at  least  240  oz.  of  silver  per 
ton.  In  1892  the  railroad  reached  the  city  of  Durango,  making 
it  profitable  to  ship  90-oz.  ore;  and  when,  in  1900,  the  branch 
line  was  opened  to  Chinacates,  60-oz.  silver-ore  could  be 
shipped  at  a  profit. 

The  mine  was  first  opened  by  the  San  Joaquin  shaft,  on  a 
hill-side  (see  Fig.  2),  and  later  by  the  Cinco  Senores  workings, 
still  further  ap  the  hill.  The  present  main  working- shaft,  the 
Refugio,  is  in  the  ravine  beside  the  hill  upon  which  the  older 
openings  are  situated.  The  shaft-house  is  visible  in  Figs.  2 
and  3. 

This  shaft  has  been  sunk  to  a  depth  of  675  ft.,  of  which  the 
first  280  ft.  are  in  the  vein,  and  14  levels  run  from  it.  Levels  1 
to  9  are  40  ft.  apart;  levels  9  to  14  are  71  ft.  apart.  Level  1  is 
a  tunnel  which  starts  at  the  mouth  of  the  shaft  and  passes  into 
the  hill  to  the  SE.  where  it  connects  with  the  early  workings 
mentioned  above.  This  level  has  the  furthest  extent  towards 
the  SE.  of  any.  The  other  levels  are  all  drifts  run  in  both  di- 
rections on  the  vein  for  longer  or  shorter  distances.  Level  9 
extends  furthest  towards  the  NW.,  passing  the  fork  in  the  Pro- 
montorio vein  into  the  West  vein  and  finally  connecting  with 
another  shaft,  the  Santa  Maria,  at  a  depth  of  250  ft.  From 
the  SE.  face  of  the  first  level  to  the  l^W.  face  of  the  ninth 
level  is  a  horizontal  distance  of  3,628  ft.  A  cross-cut  from  a 
point  in  the  Santa  Maria  ravine  a  short  distance  below  the 
Santa  Maria  shaft  intersects  the  fourth  level  and  converts  it 
into  a  tunnel. 

When  I  visited  the  mine  in  the  fall  of  1906,  the  method  of 
operation  was  as  follows :  All  the  material  mined  was  trammed 
out  to  a  patio or  sorting-yard  by  means  of  the  cross-cut 
from  the  fourth  level.  Rock  mined  above  the  fourth  level 
was  dropped  to  it,  and  rock  mined  below  was  raised  to  it  by 
the  hoist  at  the  Refugio  shaft.  On  the  patio  the  produce 
of  the  mine  was  sorted  to  "  shipping  "  ore,  running  60  oz.  of 
silver  per  ton,  and  better.    The  rejected  material  was  thrown 

[15] 


98  THE  PROMONTORTO  SILVER-MINE,  DURAXGO,  MEXICO. 

on  the  milling-clump  when  it  ran  better  than  20  oz.,  and  on 
the  waste-dump  when  worse.  Since  I  left  Promontorio,  a  mill 
has  been  started,  and  it  is  probable  that  ore  of  the  grade 
formerly  shipped  is  now  being  milled  together  with  the  lower- 
grade  material. 

The  power-house,  situated  in  a  small  arroyo,  across  from  the 
main  shaft  (see  Fig.  3),  contains  six  tubular  boilers,  which  burn 
wood  and  sapplj  steam  to  the  hoist  and  compressor,  and  to  en- 
gines operating  generators.  The  electricity  generated  supplies 
power  at  the  patio,  illuminates  electric  lamps,  and  operates  the 
pumps.  A  120-kw.  alternating-current  generator  supplies  the 
power  to  four  3-in.  Worthington  electric  pumps.  In  the  dry 
season  the  pumps  are  worked  for  8-hr.  a  day,  and  raise  about 
96,000  gal.  of  water  to  the  fourth  level  daily.  Most  of  this 
water  comes  from  the  fourteenth  level,  a  distance  of  555  ft. 
In  the  rainy  season,  it  is  necessary  to  run  the  pumps  14-hr. 
a  day,  thus  raising  168,000  gal.  per  day. 

Fire-wood  delivered  at  Promontorio  costs  $2.50  per  cord  and 
lumber  $17.50  per  1,000  ft.    The  wages  paid  per  day  are  : 


.  §0.75 

Laborers,  ..... 

0.375  to  10.75 

.  1.25 

Shift  Bosses,  

.     1.00  to  1.25 

Timbermen  and  Carpenters, 

0.875 

Blacksmiths,  ..... 

.  1.25 

.  1.25 

.  1.00 

Firemen,  ..... 

.  0.625 

Labor  is,  on  the  whole,  plentiful.  Some  difficulty  is  experi- 
enced in  keeping  sufficient  men  at  the  mine  during  the  periods 
of  sowing  and  harvesting  of  crops,  for  the  Durango  laborer 
prefers  farming  to  mining.  Another  peculiar  condition  has  to 
be  met  in  cold  weather,  when  all  hands  want  to  work  on  night- 
shift  because  their  cabins  are  too  cold  to  sleep  in  at.  night,  but 
are  warmed  to  a  more  comfortable  temperature  by  the  sun 
during  the  day.  With  the  exercise  of  a  little  tact  there  should 
be  no  difficulty  in  obtaining  all  the  labor  desired. 


[16] 


THE  PROMONTORIO  SILVER-MINE,  DURANGO,  MEXICO.  99 


YIIL  Milling. 

Milling  operations  did  not  prove  a  great  success  at  Promon- 
torio  in  the  early  days  of  the  mine.  One  brand-new  mill,  which 
had  never  turned  a  wheel,  was  completely  destroyed  by  the  col- 
lapse of  a  dam.  Another  mill  was  erected  later,  but  failed  to 
give  good  extractions,  as  the  tailings-dumps  bear  witness. 

Mr.  Gordon  Wilson  carried  on  a  long  series  of  milling-tests 
upon  Promontorio  ore,  and  came  to  the  conclusion  that  the  best 
results  could  be  obtained  by  concentration,  followed  by  sliming 
and  cyanidation  of  the  tails.  His  experiments  indicated  that 
45  per  cent,  of  the  values  could  be  extracted  by  concentration 
in  10  per  cent,  of  the  weight,  and  that,  of  the  remaining  55  per 
cent.,  89  per  cent,  of  the  silver,  and  practically  all  the  gold, 
could  be  extracted  by  sliming  and  cyaniding  for  a  period  of  10 
days.  This  would  mean  a  total  extraction  of  94  per  cent.  Mr. 
Wilson,  therefore,  constructed  a  50-metric-ton  concentrating 
and  cyaniding  mill  at  Santa  Ines,  which  has  now  been  in  oper- 
ation for  several  months,  and,  I  am  informed,  is  performing 
work. 

The  dam  which  supplies  water  for  this  mill  is  situated  mid- 
way between  Promontorio  and  Santa  Ines.  It  is  100  ft.  high, 
and  was  constructed  at  a  cost  of  $50,000. 

IX.  Production. 

The  smelter-returns  on  shipments  from  Promontorio  are  re- 
ported to  have  been  about  $5,000,000.  Unfortunately,  no  rec- 
ords of  the  quantities  of  gold  and  silver  produced  were  kept 
during  the  early  bonanza  days  of  the  mine.  From  Dec.  5, 1896, 
to  Aug.  18,  1906,  there  were  produced  and  sold  5,689,618  oz. 
of  silver  and  15,857.4  oz.  of  gold.  During  this  period  of  re- 
corded production  the  Promontorio  mine  lost  its  position  as 
"  one  of  the  largest  producers  of  silver  in  Mexico,"^  and  became 
one  of  the  many  minor  producers.  At  present  there  is  but  little 
shipping-ore  in  sight  in  the  mine,  but  there  are  large  reserves 
of  good  milling-ore,  and,  with  new  and  effective  milling  methods 
in  full  operation,  we  may  soon  expect  to  see  the  Promontorio 
mine  make  its  way  to  the  front  once  more. 


3  In  galls,  Trans.,  xxv.,  149  (1895). 
[17] 


